From German patent application 24 20 546, it is known to coat furnaces which are made of a graphite tube with a layer of pyrocarbon in carrying out electro-thermal decomposition of samples. Thereby, in contrast to pure graphite, a non-porous surface is provided by such coating and the sample liquid is prevented from infiltrating the graphite. If such infiltration of sample liquid into the graphite tube were permitted, the atomization of elements of the sample is delayed. Thereby, the sensitivity and the accuracy of the measurement suffer.
Pyrocarbon is a carbon which is generated by thermal decomposition of carburent on the surface of a graphite element, i.e., here a graphite tube. Different methods of supplying a layer of pyrocarbon onto the surface of a graphite tube are known. Such a method is described in German patent application 24 20 546. Other methods work with gaseous carburents such as methane. In vacuo, where the elements which are to be coated are arranged, argon as a carrier gas carries methane over the surfaces which are to be coated, the methane being decomposed there generates the pyrocarbon coating.
In practice, it is often difficult to achieve uniform coating of graphite furnaces with pyrocarbon. This is particularly true for the interior of the cavity into which the sample for the atomization is introduced, i.e., the bore of a graphite tube. The thickness of the layer is difficult to determine. Usually, the thickness of the layer being too small becomes obvious only in practical operation, namely by the early deterioration of the analytical results. The pyrocarbon layer wears out by use, so that, if a layer of pyrocarbon is too small, after relatively few analyses, the sample liquid, as described, will seep away in the material of the furnace which is no longer sufficiently coated.
The "graphite tube atomizer" in which the graphite furnace is formed by a graphite tube through which current flows in its longitudinal direction is described in German patent application 27 18 416.
EP-A2-0 311 761 shows a furnace for the electro-thermal atomization of samples with a tubular furnace body which is provided on opposite sides with longitudinal contact ribs which are, in turn, integral with cylindrical contact pieces. In the bore of the furnace body hollow, generally semi-cylindrical platforms extending through approximately 180.degree. are provided. This platform is integral with the furnace body, i.e., it forms with the furnace body, with the contact ribs, and the contact pieces an integral element made of graphite. Opposite the platform, the furnace body has a sample inlet opening.
It is the purpose of the platform to delay the heating of the sample supplied to the platform to atomization temperature as compared to the heating of the wall of the furnace so that there is already a thermal equilibrium when the sample is finally atomized. This is achieved in that the sample on the platform is heated substantially indirectly by heat radiation from the wall of the furnace.
From German utility model 89 01 529.0, a furnace similar to that described above is known in which the hollow, generally semi-cylindrical platform being integral with the furnace is connected to the furnace body through a narrow web which is arranged on the one side of the platform and only on one side of the longitudinal center plane of the platform.
When the furnaces are produced according to EP-A-0 311 761 or according to German utility model 89 01 529.0, there is relatively much refuse when the integral platform is machined. In addition, very particular problems occur with the coating with pyrocarbon of such furnaces having a complex structure.